Polyphenylene sulfide films have excellent mechanical properties and heat resistance, so that various uses thereof are expected and have been proposed. For example, Japanese laid-open patent application (Kokai) No. 57-121052 discloses a non-oriented polyphenylene sulfide sheet, and Japanese laid-open patent application (Kokai) No. 54-142275 discloses a biaxially oriented polyphenylene sulfide film. Although the non-oriented sheets have high tear strengths, if they are exposed to a high temperature, their mechanical strengths are sharply decreased, and they have poor long term heat resistances. On the other hand, biaxially oriented films have much improved mechanical properties other than tear strength, and their chemical resistances and long term heat resistances are extremely high. However, as is apparent from the fact that they have excellent chemical resistances and heat resistances, since their surfaces are inert, they have poor adhesiveness. Therefore, in order to use them for wrapping, electrical insulation or for general industrial applications, it is necessary to give adhesiveness, particularly heatsealability.
As a method for promoting adhesiveness, it is known to perform corona discharge treatment (Japanese laid-open patent application (Kokai) No. 57-187327). However, with this method, although the adhesiveness is slightly improved, it is insufficient. Further, improvement in heatsealability is scarcely observed. It has also been proposed to apply an adhesive. However, not only does the adhesive per se have poor heat resistance, but also the deterioration of the adhesive by heat accelerates the deterioration of the polyphenylene sulfide film, so that the heat resistance is extremely poor.
To overcome the drawback of the biaxially oriented polyphenylene sulfide films that their tear strengths are poor, a film in which a copolymerization unit other than p-phenylene sulfide unit is copolymerized has been disclosed (Japanese laid-open patent application (Kokai) No. 61-98526). However, although the improvement in the tear strength of the film is observed, other mechanical properties are deteriorated.